Ned,
Buy & read that options trading 101 book I mentioned. It will answer a lot of questions. Short answer is to buy as much time as a specific play requires for the amount of risk you're willing to take as well as your conviction in the likelihood of what you expect the stock to do happening.
Normally if I buy a long option, I'm buying with it being slightly in the money, and expecting it to rise to make me a decent profit within the medium term, 1 - 4 months, so I buy usually 3 or 4 months out.
That's just a general guideline. There is so much more that can be said on playing time with options. It's a whole new vector that changes based on how much time is left, and how much time you originally paid or were paid for in the term of writing contracts.